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Astronomy 161
An Introduction to Solar System Astronomy
Prof. Scott Gaudi

Lecture 25: Measuring Light:

Key Ideas:

Every atom, ion, and molecule has a unique spectral signature.

Absorption and Emission of Photons


Looking inside the Atom

Electrons cannot orbit just anywhere around a nucleus: The details are dictated by quantum mechanics.

Hydrogen: The Simplest Atom

An atom of Hydrogen (1H) consists of:

First orbital: Ground State (n=1)

Higher orbitals: Excited States (n=2,3,...)

Emission & Absorption Lines

Emission Lines:

When an electron jumps from a higher to a lower energy orbital, a single photon is emitted with exactly the energy difference between orbitals. No more, no less.

Electrons can get into the excited states by either

Absorption Lines:

When an electron absorbs a photon with exactly the energy needed to jump from a lower to a higher orbital.

Absorption is very specific:

The excited states decay by emitting photons in random directions.


If an atom or molecule absorbs enough energy from a photon or a collision, an electron can be ejected. Similarly, you can also add extra electrons: Ions differ from their parent neutral atoms or molecules:

Fingerprinting Matter

Other atoms have more electrons, and hence more complex electron orbital structures.

Every element has its own, distinctive spectral signature.


Molecules are more complex still:

Results in very complex spectra:

Molecules mainly produce strong lines at infrared, microwave, and radio wavelengths.

The Importance of Spectroscopy

From the emission or absorption lines in an object's spectrum, we can learn: These data give us a nearly complete picture of the physical conditions in the object.

Spectroscopy is one of the most important tools of the astronomer.

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